Lynch syndrome (LS) is a common cause of hereditary colorectal cancer (CRC). Some CRC
patients develop mismatch repair deficiency without germline pathogenic mutation, known as
Lynch-like syndrome (LLS). We compared the risk of CRC in first-degree relatives (FDRs) in LLS and
LS patients. LLS was diagnosed when tumors showed immunohistochemical loss of MSH2, MSH6,
and PMS2; or loss of MLH1 with BRAF wild type; and/or no MLH1 methylation and absence of
pathogenic mutation in these genes. CRC and other LS-related neoplasms were followed in patients
diagnosed with LS and LLS and among their FDRs. Standardized incidence ratios (SIRs) were
calculated for CRC and other neoplasms associated with LS among FDRs of LS and LLS patients.
In total, 205 LS (1205 FDRs) and 131 LLS families (698 FDRs) had complete pedigrees. FDRs of
patients with LLS had a high incidence of CRC (SIR, 2.08; 95% confidence interval (CI), 1.56–2.71),
which was significantly lower than that in FDRs of patients with LS (SIR, 4.25; 95% CI, 3.67–4.90;
p < 0.001). The risk of developing other neoplasms associated with LS also increased among FDR
of LLS patients (SIR, 2.04; 95% CI, 1.44–2.80) but was lower than that among FDR of patients with
LS (SIR, 5.01, 95% CI, 4.26–5.84; p < 0.001). FDRs with LLS have an increased risk of developing
CRC as well as LS-related neoplasms, although this risk is lower than that of families with LS. Thus,
their management should take into account this increased risk